1. Field of the Invention
The present invention relates to a data communication apparatus having an auto-calling function.
2. Related Background Art
A facsimile apparatus having an auto-dialing function has been known.
In the communication by the facsimile apparatus having an auto-dialing (hereinafter referred to as AD) function, an acknowledge signal from a destination station is monitored for only a time T1 (35-40 seconds) as defined by the CCITT Recommendation T.30. A timing to set the time T1 is usually after dialing or after detection of a polarity reversal of a line.
In a facsimile apparatus having an automatic responding and recording telephone set connected thereto, the apparatus does not immediately acknowledges a calling signal but waits until the automatic responding and recording telephone set starts to operate. After the automatic responding and recording telephone set has started the operation, the apparatus monitors a line to determine if a calling station is a person or a facsimile apparatus., and when it detects a calling signal (CNG) which indicates that the calling station is a facsimile apparatus, it stops the operation of the automatic responding and recording telephone set and sends out a protocol signal to carry out facsimile communication.
Such a facsimile apparatus is disclosed in U.S. Pat. Nos. 4,800,439 and 4,932,048 and U.S. patent application Ser. No. 403,842 refiled on Sep. 5, 1989.
In order for a called facsimile apparatus to detect a CNG signal while it sends out an acknowledgement message, it is necessary to insert a frequency band elimination filter in an acknowledgement message transmission unit but this causes a problem in view of a cost and a circuit board packaging space. Accordingly, many of such apparatus determine the detection of the CNG signal during a message recording period following to the transmission of the acknowledgement message.
However, the method of setting of the time T1 after the detection of the polarity reversal of the line will fall into disuse because there exist exchanges which do not reverse the polarity.
In the facsimile apparatus which detects the CNG signal after the automatic responding and recording telephone set has been started in response to a call and an acknowledgement signal has been sent out, a response to the CNG signal is delayed. Accordingly, where the time T1 is set to monitor the response, there is a risk that the time T1 has already been elapsed when the called station sends back the acknowledgement signal and the communication may not be established because the line has been opened.